Recombinant yeast and a method for producing ethanol using the same

Abstract

An acetic acid metabolizing ability of a recombinant yeast strain having xylose-metabolizing ability is to be improved. In such a recombinant yeast strain having xylose-metabolizing ability, the acetaldehyde dehydrogenase gene has been introduced and a gene encoding NADH dehydrogenase involved in reoxidation of cytoplasmic NADH on the mitochondrial outer membrane has been suppressed.

What is claimed is: 
     
       1. A recombinant yeast strain having xylose-metabolizing ability comprising an acetaldehyde dehydrogenase gene introduced thereinto, wherein a gene encoding NADH dehydrogenase involved in reoxidation of cytoplasmic NADH on the mitochondrial outer membrane is suppressed, and
 wherein in said recombinant yeast strain, the only gene(s) that are suppressed as compared to a corresponding wild-type yeast consist of: (a) said NADH dehydrogenase; or (b) said NADH dehydrogenase and an alcohol dehydrogenase gene. 
 
     
     
       2. The recombinant yeast strain according to  claim 1 , wherein the gene encoding NADH dehydrogenase encodes a protein (a) or (b) below:
 (a) a protein comprising the amino acid sequence as shown in SEQ ID NO: 2 or 4; or 
 (b) a protein comprising an amino acid sequence exhibiting 70% or higher identity with the amino acid sequence as shown in SEQ ID NO: 2 or 4 and having enzymatic activity of catalyzing a reaction of converting NADH into NAD + . 
 
     
     
       3. The recombinant yeast strain according to  claim 1  comprising a xylose isomerase gene introduced thereinto. 
     
     
       4. The recombinant yeast strain according to  claim 3 , wherein the xylose isomerase gene encodes a protein (a) or (b) below:
 (a) a protein comprising the amino acid sequence as shown in SEQ ID NO: 6; or 
 (b) a protein comprising an amino acid sequence exhibiting 70% or higher identity with the amino acid sequence as shown in SEQ ID NO: 6 and having enzymatic activity of converting xylose into xylulose. 
 
     
     
       5. The recombinant yeast strain according to  claim 1 , which further comprises a xylulokinase gene introduced thereinto. 
     
     
       6. The recombinant yeast strain according to  claim 1 , which comprises a gene encoding an enzyme selected from a group of enzymes constituting a non-oxidative process in the pentose phosphate pathway introduced thereinto. 
     
     
       7. The recombinant yeast strain according to  claim 6 , wherein the group of enzymes constituting a non-oxidative process in the pentose phosphate pathway includes ribose-5-phosphate isomerase, ribulose-5-phosphate-3-epimerase, transketolase, and transaldolase. 
     
     
       8. The recombinant yeast strain according to  claim 1 , in which an alcohol dehydrogenase gene having activity of converting acetaldehyde into ethanol is introduced. 
     
     
       9. The recombinant yeast strain according to  claim 1 , wherein said alcohol dehydrogenase gene is disrupted. 
     
     
       10. A method for producing ethanol comprising a step of culturing the recombinant yeast strain according to  claim 1  in a xylose-containing medium to perform ethanol fermentation. 
     
     
       11. The method for producing ethanol according to  claim 10 , wherein the medium contains cellulose and the ethanol fermentation proceeds simultaneously with saccharification by at least a cellulase.